$min and $max update operators in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the document after applying $min update?

Given the document

{ "_id": 1, "score": 50 }

and the update operation

{ "$min": { "score": 40 } }

, what will be the value of score after the update?

MongoDB

db.collection.updateOne({ _id: 1 }, { $min: { score: 40 } })
db.collection.findOne({ _id: 1 })

A { "_id": 1, "score": 40 }

B { "_id": 1, "score": null }

C { "_id": 1, "score": 90 }

D { "_id": 1, "score": 50 }

💡 Hint

The $min operator updates the field only if the specified value is less than the current value.

The $min operator updates the field only if the new value is less than the current value. Since 40 is less than 50, the score is updated to 40.

❓ query_result

intermediate

What is the document after applying $max update?

Given the document

{ "_id": 2, "level": 10 }

and the update operation

{ "$max": { "level": 15 } }

, what will be the value of level after the update?

MongoDB

db.collection.updateOne({ _id: 2 }, { $max: { level: 15 } })
db.collection.findOne({ _id: 2 })

A { "_id": 2, "level": 15 }

B { "_id": 2, "level": 10 }

C { "_id": 2, "level": 5 }

D { "_id": 2, "level": null }

💡 Hint

The $max operator updates the field only if the specified value is greater than the current value.

The $max operator updates the field only if the new value is greater than the current value. Since 15 is greater than 10, the level is updated to 15.

📝 Syntax

advanced

Which update operation is syntactically correct to use $min on nested field?

You want to update the nested field stats.score to 30 only if 30 is less than the current value. Which of the following update operations is correct?

A { "$min": { "stats": { "score": 30 } } }

B { "$min": { stats: { score: 30 } } }

C { "$min": { "stats.score": 30 } }

D { "$min": { "stats->score": 30 } }

💡 Hint

Use dot notation to update nested fields in MongoDB.

MongoDB uses dot notation to specify nested fields in update operations. Option C correctly uses "stats.score" as the field name.

❓ query_result

advanced

What happens if $max is used with a value less than current field value?

Given the document

{ "_id": 3, "points": 100 }

and the update

{ "$max": { "points": 80 } }

, what will be the value of points after the update?

MongoDB

db.collection.updateOne({ _id: 3 }, { $max: { points: 80 } })
db.collection.findOne({ _id: 3 })

A { "_id": 3, "points": 80 }

B Update operation fails with error

C { "_id": 3, "points": null }

D { "_id": 3, "points": 100 }

💡 Hint

The $max operator only updates if the new value is greater than the current value.

The $max operator updates the field only if the new value is greater than the current value. Since 80 is less than 100, the field remains unchanged.

🧠 Conceptual

expert

Why would you use $min and $max operators in a leaderboard system?

In a game leaderboard, you want to keep track of the highest and lowest scores a player has achieved. Which statement best explains why $min and $max operators are useful here?

A $min and $max operators automatically sort the leaderboard by scores.

B $min and $max allow atomic updates to keep track of lowest and highest scores without reading the document first.

C $min and $max operators create new fields for lowest and highest scores automatically.

D $min and $max operators delete scores that are not minimum or maximum.

💡 Hint

Think about how to update values safely when multiple users might update scores at the same time.

The $min and $max operators allow atomic updates to a field only if the new value is less or greater respectively. This avoids race conditions and the need to read the document before updating.

Practice

1. What does the $min update operator do in MongoDB?

easy

A. Updates the field only if the new value is smaller than the current value.

B. Updates the field only if the new value is larger than the current value.

C. Always updates the field with the new value.

D. Deletes the field if the new value is smaller.

Solution

1. Step 1: Understand the purpose of $min

   The $min operator updates a field only if the new value is smaller than the current value stored in that field.

2. Step 2: Compare with other options

   $max updates if the new value is larger, and other options do not match the behavior of $min.

3. Final Answer:

   Updates the field only if the new value is smaller than the current value. -> Option A

4. Quick Check:

   $min updates only if smaller [OK]

Hint: Remember: $min means update if new value is smaller [OK]

Common Mistakes:

• Confusing $min with $max behavior
• Thinking $min always updates the field
• Assuming $min deletes fields

2. Which of the following is the correct syntax to use $max to update the field score to 90 in MongoDB?

easy

A. { $max: [ score: 90 ] }

B. { $max: score: 90 }

C. { $max: { score: 90 } }

D. { $max: { 'score' => 90 } }

Solution

1. Step 1: Recall correct MongoDB update syntax

   MongoDB update operators use the format: { $operator: { field: value } }. So $max must be followed by an object with field and value.

2. Step 2: Check each option

   { $max: { score: 90 } } matches the correct syntax. The options missing braces around the field-value pair or using brackets [] have invalid syntax. { $max: { 'score' => 90 } } uses '=>' which is not valid in MongoDB queries.

3. Final Answer:

   { $max: { score: 90 } } -> Option C

4. Quick Check:

   Correct MongoDB update syntax = { $max: { score: 90 } } [OK]

Hint: Use braces { } around field-value pairs in update operators [OK]

Common Mistakes:

• Using brackets [] instead of braces {}
• Missing braces around field-value pairs
• Using arrow syntax '=>' instead of colon ':'

3. Given a document { _id: 1, highScore: 80 }, what will be the document after applying db.collection.updateOne({_id:1}, { $max: { highScore: 75 } })?

medium

A. { _id: 1, highScore: 75 }

B. { _id: 1, highScore: 155 }

C. No document updated

D. { _id: 1, highScore: 80 }

Solution

1. Step 1: Understand $max behavior on existing value

   The $max operator updates the field only if the new value is greater than the current value. Here, current highScore is 80, new value is 75.

2. Step 2: Compare values and decide update

   Since 75 is less than 80, the field will NOT be updated. The document remains unchanged.

3. Final Answer:

   { _id: 1, highScore: 80 } -> Option D

4. Quick Check:

   $max updates only if new value is larger [OK]

Hint: If new value < current, $max does not update [OK]

Common Mistakes:

• Assuming $max always updates the field
• Confusing $max with $min behavior
• Thinking update fails if condition not met

4. You run this update: db.collection.updateOne({_id: 2}, { $min: { age: 30 } }) but it does not update the document. What could be the reason?

medium

A. The update syntax is incorrect.

B. The current age in the document is less than 30.

C. The document with _id: 2 does not exist.

D. The $min operator requires an array value.

Solution

1. Step 1: Understand $min update condition

   $min updates the field only if the new value is smaller than the current value.

2. Step 2: Analyze why update might not happen

   If the current age is already less than 30, the update will not change the document. Other options are incorrect because $min does not require arrays, the syntax is correct, and the question states the update runs but no change occurs.

3. Final Answer:

   The current age in the document is less than 30. -> Option B

4. Quick Check:

   $min updates only if new value smaller [OK]

Hint: No update if current value is smaller than $min value [OK]

Common Mistakes:

• Thinking $min requires array values
• Assuming syntax error without checking
• Ignoring existing document values

5. You want to keep track of the lowest and highest temperatures recorded in a MongoDB document with fields minTemp and maxTemp. Which update operation correctly updates both fields when a new temperature temp is 15?

hard

A. { $min: { minTemp: 15 }, $max: { maxTemp: 15 } }

B. { $max: { minTemp: 15 }, $min: { maxTemp: 15 } }

C. { $min: { maxTemp: 15 }, $max: { minTemp: 15 } }

D. { $set: { minTemp: 15, maxTemp: 15 } }

Solution

1. Step 1: Understand the goal for minTemp and maxTemp

   We want minTemp to update only if the new temperature is lower, so use $min. For maxTemp, update only if new temperature is higher, so use $max.

2. Step 2: Check each option for correct operator usage

   { $min: { minTemp: 15 }, $max: { maxTemp: 15 } } correctly uses $min for minTemp and $max for maxTemp. { $max: { minTemp: 15 }, $min: { maxTemp: 15 } } and { $min: { maxTemp: 15 }, $max: { minTemp: 15 } } swap operators incorrectly. { $set: { minTemp: 15, maxTemp: 15 } } always sets both fields to 15, ignoring previous values.

3. Final Answer:

   { $min: { minTemp: 15 }, $max: { maxTemp: 15 } } -> Option A

4. Quick Check:

   Use $min for minTemp and $max for maxTemp [OK]

Hint: Use $min for minimum and $max for maximum fields [OK]

Common Mistakes:

• Swapping $min and $max operators
• Using $set which overwrites values blindly
• Updating wrong fields with wrong operators